(a) Field of the Invention
The present invention relates to a power supply. In particular, the present invention relates to a power supply including a bleed switch for an LED lamp which is controlled by a TRIAC dimmer.
(b) Description of the Related Art
The TRIAC dimmer is widely used for controlling a current provided to a load. Referring to FIG. 1 and FIG. 2, the TRIAC dimmer will be described.
FIG. 1 is a schematic diagram representing a prior circuit including a TRIAC dimmer, and FIG. 2 is a waveform diagram showing an operation of the TRIAC dimmer.
As shown in FIG. 1, the TRIAC dimmer 10 is connected in series to an AC power source Vac and a load 20. The TRIAC dimmer 10 includes a variable resistor R1, a resistor R2, a capacitor C1, a DIAC, and a TRIAC.
The AC power source Vac provides a voltage source with an AC waveform as shown in FIG. 2 (A). An RC network consisting of the variable resistor R1, the resistor R2, and the capacitor C1 delays turn-on of the TRIAC until a voltage of the capacitor C1 reaches a trigger voltage of the DIAC. From this, two terminals of the load 20 are provided with a voltage with a waveform as shown in FIG. 2 (B).
In the other hand, if resistance of the variable resistor R1 increases, the delay of turn-on of the TRIAC also increases. That is, because on-time of the TRIAC reduces, an average power provided to the load 20 reduces.
In this way, when the TRIAC dimmer 10 is used, a power provided to the load 20 can be controlled by controlling a value of the variable resistor R1.
In particular, in a case that the load 20 corresponds to an AC-DC converter and an LED lamp, if on-time of the TRIAC is shorter than a certain time, flicker can be caused. A constant current is required to continuously flow to prevent this problem. This current is called a bleed current.
However, efficiency of the AC-DC converter worsens because of the continuous current.